This invention relates to the structure and fabrication of magnetic devices having a spin-valve structure.
The giant magnetoresistance (GMR) effect is known in the art. It is of large interest for applications in magnetoresistive read and write heads and for magnetic sensors. GMR magnetic multilayers exhibit strong coupling characteristics and thus high saturation fields. A multilayer or superlattice is a multitude of ferromagnetic and antiferromagnetic layers. The ferromagnetic layers can be coupled or uncoupled.
Recently several research groups have shown that the single bath electrodeposition or electrodeposition technique, is an attractive technique for the production of GMR multilayers (W. Schwarzacher and D. S. Lashmore, Giant magnetoresistance in electrodeposited films, IEEE-Trans. Magn., 32 (1996) 3133-53). Due to the above mentioned high saturation fields, these electrodeposited/electroplated multilayers are unattractive for low field sensing. The largest sensitivity so far obtained in electrodeposited multilayers has been 0.1% per Oe (R. Hart, M. Alper, K. Attenborough and W. Schwarzacher, Giant magnetoresistance in Nixe2x80x94Coxe2x80x94Cu/Cu superlattices electrodeposited on n-type (100) GaAs substrates, Proc. 3rd International Symposium on Magnetic Materials, Processes and Devices, Electrochem. Soc. Proc., 94 (1994) 215-221). If electrodeposition is to be considered as a suitable production technique for future magnetic field sensing devices, higher sensitivity at lower switching fields must therefore be achieved.
Electrodeposition or electroplating in the art usually takes place on a conductive seed layer, typically Cu. If this seed layer cannot be removed, problems of current shunting and signal loss occur when measuring the magnetoresistive properties of the electrodeposited or electroplated materials.
New magnetic field sensors having a spin-valve structure have been disclosed. These structures have the advantage of high magnetoresistance (MR) signals and high sensitivities at lower fields (J. C. S. Kools, Exchange-Biased Spin-Values for Magnetic Storage, IEEE Trans. Magn., 32 (1996) 3165-3184). Such combination of features is of importance for many sensing applications. The production of films, showing these effects, is usually achieved by sputtering techniques.
A spin-valve is a structure with a thin non-magnetic spacer layer sandwiched between two ferromagnetic (FM) layers, which have different coercivities. The coercivity of a magnetic material reflects the resistance to a change of the orientation of the magnetic field when a magnetic (or electric) field is externally applied. In sputtered spin-valves this can be achieved by having different layer thickness, different materials or by pinning one of the layers to an antiferromagnetic (AF) layer, leaving the other magnetic layer free to rotate. The magnetisation alignment of the ferromagnetic layers can be changed from antiparallel (high resistance state) to parallel (low resistance state) depending on the externally applied magnetic field.
A recent advance in spin-valve design used to replace the typical AF materials, e.g. FeMn or NiO, has been the implementation of an artificial or synthetic antiferromagnetic subsystem (AAF or SyAF)(H. A. M van den Berg, W. Clemens, G. Gieres, G. Rupp, W. Schelter and M. Vieth, GMR sensor scheme with artificial antiferromagnetic subsystem, IEEE Trans. Magn., 32 (1996) 4624-4626)) (J. L. Leal and M. H. Kryder, Spin-valves exchange biased by Co/Ru/Co synthetic antiferromagnets, J.Appl.Phys. 83, 3720 (1997)). These new spin-valve structures are comprised of a FM layer separated by a Cu spacer the AAF subsystem which itself consists of a few strongly coupled bilayers or multilayers such as Co/Cu bilayers or Co/Ru/Co respectively. They have an added advantage over the AF layers of improved resistance against corrosion and higher processing temperatures.
A specific spin-valve structure is disclosed in document WO98/14793.
A first aim of the present invention is to provide novel magnetic devices capable of high magnetoresistive signals and high sensitivities at low fields.
Another aim of the present invention is to provide a novel magnetic device of which the properties can be determined by varying its construction parameters, preferably by appropriate choice of a carrier substrate, and to provide a novel magnetic device that can be used for several purposes by varying its mode of operation.
Another aim of the present invention is to provide new magnetic memory devices that can show multi-value memory capacity.
A further aim of the present invention is to provide a novel method for producing a magnetic sensor capable of sensing high magnetoresistive signals and having high sensitivities at low fields.
Another aim of the present invention is to provide a novel low-cost fabrication method for spin-valve structures.
The present invention concerns in a first aspect a spin-valve structure comprising a first and a second free ferromagnetic layer and a spacer layer positioned between said first and second free ferromagnetic layers, and wherein said first free ferromagnetic layer is positioned on a substrate. In a preferred embodiment of the invention, said first free ferromagnetic layer is in direct contact with the surface of said substrate.
Advantageously, said spacer layer is an antiferromagnetic or metal or semimetal or conductive semiconductor layer or any combination thereof or any multiple layer structure of any of these layers.
In the free ferromagnetic layers, the orientation of the magnetic fields can be changed, preferably independently one of the other, with an externally applied electrical or magnetic field. The first and second free ferromagnetic layers, in a preferred embodiment of the present invention, can be switched at different externally applied electrical or magnetic fields. One of the free layers can have a parallel behaviour and one of the free layers can have an antiparallel behaviour.
Preferably, the spacer layer can be an artificial antiferromagnetic layer or a synthetic antiferromagnetic layer.
The artificial antiferromagnetic layer can comprise Cu layers and Co layers positioned therebetween, said Cu layers being thin enough as to increase magnetic coupling between said Co layers. These layers or any CoxNiyFe1xe2x88x92xxe2x88x92y alloy or any CoxFeyX1xe2x88x92xxe2x88x92y alloy (X being Cr, V, Ni, Cu) can act as a single hard layer.
Said first and/or second free ferromagnetic layer can comprise Co, but can also comprise NiFe or CoFe. Preferably, the first free ferromagnetic layer that is positioned on said semiconductor substrate has a higher coercivity than the other free ferromagnetic layer.
Preferably, the substrate is a semiconductor substrate. Said semiconductor substrate can be a GaAs or Ge or Si or a polymer or any other semiconducting substrate.
Said substrate can be either a semiconductor (including GaAs and Si) or any substrate that is conductive enough to allow plating but some barrier is to be formed so that in the sensing operation the sensing current does not enter the substrate. The substrate choice can also include a semiconducting polymer, or a polymer that makes a Schottky or other Barrier contact with a metal deposited thereon. In an alternative, a highly ohmic layer should be deposited first on the substrate. Thus, the ferromagnetic layer can be separated from the substrate by an insulating tunnelling barrier.
The magnetic and structural properties of said first free ferromagnetic layer can be influenced by the structure of the surface and/or the lattice structure of the semiconductor substrate on which it is positioned. Foregoing the deposition of the first free ferromagnetic layer on the substrate, a change of the semiconductor surface structure by ion bombardment or any other method may change the magnetic and structural properties of the first free ferromagnetic layer that is grown thereon.
In a preferred embodiment, there is an electrical barrier between the first free ferromagnetic layer and the semiconductor surface. Said electrical barrier can be a Schottky Barrier or a Tunnel Barrier. Such electrical barrier can be formed while depositing said first free ferromagnetic layer on said semiconductor surface and can prevent shunting currents and protect said spin-valve structure against electrostatic discharge. This barrier however is weak enough to provide electrical contact if necessary during deposition.
Said spin-valve structure can act as a magnetic memory device, preferably a magnetic memory device having more than two memory states. Said magnetic memory device can be set using current or voltage pulses of predefined magnitude and pulse width. For achieving a different memory setting, the pulses can be of a fixed magnitude and a variable pulse width, or the pulses can be of a variable magnitude and a fixed pulse width.
The present invention also relates to a spin-valve structure comprising a first and a second free ferromagnetic layers and a spacer layer, preferably an antiferromagnetic or a metal or semimetal or conductive semiconductor layer, positioned between said first and said second free ferromagnetic layers wherein said first free ferromagnetic layer is positioned on a substrate forming a barrier with a ferromagnetic layer being positioned thereon. This barrier is such that it can prevent shunting currents or other forms of electrical transport from the substrate to the magnetic layers during operation of the spin-valve structure while allowing for electrical contact if necessary during deposition of the ferromagnetic layer on the substrate. Such substrate can also be a polymer material.
Yet according to the present invention, also a spin-valve structure is disclosed, comprising a first and a second free ferromagnetic layers and an antiferromagnetic or a metal or semimetal or conductive semiconductor or insulating layer positioned between said first and said second free ferromagnetic layers, wherein said first free ferromagnetic layer is positioned on a conducting substrate with an insulating layer therebetween.
The insulating layer can be such that it is providing a barrier weak enough for having electrical transport during deposition of the first free ferromagnetic layer preferably by electroplating and high enough for confining electrical currents in the layers during operation of the spin-valve structure. During operation, the barrier is to act as a Schottky Barrier or a Tunnel Barrier or a High Ohmic Contact.
Further is disclosed, according to the present invention, a spin-valve structure comprising a first and a second free ferromagnetic layers and a spacer layer positioned between said first and said second free ferromagnetic layers, wherein said first free ferromagnetic layer is positioned on a substrate, said first free ferromagnetic layer having the same lattice structure as said substrate. Preferably, said spacer layer is one of the group of a metal layer, an antiferromagnetic layer, an insulating layer or any combination thereof or any multilayer structure of any of the layers of said group.
A second aspect of the present invention is a method for producing a spin-valve structure, comprising the step of electrodeposition or electroplating of said spin-valve structure on a substrate, preferably a semiconductor substrate. Said step of electrodeposition preferably comprises the following steps:
electrodeposition of a first ferromagnetic layer on a semiconductor substrate,
electrodeposition of a spacer layer, preferably a first antiferromagnetic or metal or semimetal or conductive semiconductor layer on the first ferromagnetic layer, and
electrodeposition of a second ferromagnetic layer on the antiferromagnetic layer.
According to a preferred embodiment, the method of the present invention can also comprise the steps of:
electrodeposition of a first ferromagnetic layer on a semiconductor substrate,
electrodeposition of a first nonmagnetic layer on the first ferromagnetic layer,
electrodeposition of an antiferromagnetic or metal or semimetal or conductive semiconductor layer on said first nonmagnetic layer,
electrodeposition of a second nonmagnetic layer on the antiferromagnetic layer,
electrodeposition of a second ferromagnetic layer on the second nonmagnetic layer.
Yet in another preferred embodiment of the invention, the method can comprise the steps of:
making an insulating layer on a conductive substrate,
electrodepositing a first ferromagnetic layer on said insulating layer,
electrodepositing a nonmagnetic layer on said first ferromagnetic layer,
electrodepositing a second ferromagnetic layer on said nonmagnetic layer.
More preferably, said insulating layer is made in an electroplating step.
In a preferred embodiment, said electrodeposition steps are performed in a single electrolyte bath. Said electrolyte can comprises several elements, said elements being selected to be deposited by an applied electrodeposition voltage.
The method can further comprise a selection or a changing step of the surface structure of said semiconductor substrate prior to the electrodeposition step. The surface structure of the semiconductor substrate can be changed by ion bombardment or any other method known in the art.
A third aspect of the present invention is the advantageous use of the spin-valve structure of the present invention for a number of applications that are mentioned below.
An aspect of the invention is the use of the spin-valve structure as described herein or obtainable by the method as described herein as a sensing element for contactless position, angle and/or movement sensing.
Another aspect of the use of the present invention is the use of any spin-valve structure as described herein or obtainable by any method as described herein as a sensing element for angular position sensing.
Another aspect of the use of the present invention is the use of any spin-valve structure as described herein or obtainable by any method as described herein as a sensing element for indirect measurement of physical parameters through the change in resistance of the multilayer structure.
Another aspect of the use of the present invention is the use of any spin-valve structure as described herein or obtainable by any method as described herein as a magnetic device in a magnetic memory circuit for building a Magnetic Random Access Memory. Said magnetic device can have a multivalue magnetic memory.
Another aspect of the use of the present invention is the use of any spin-valve structure as described herein or obtainable by any method as described herein as an element of logic gates comprised in a logic device.
Another aspect of the use of the present invention is a method of operating any spin-valve structure comprising any barrier as described herein, whereby currents are confined in-plane by said barrier.
Yet another aspect of the use of the present invention is a method of operating any spin-valve structure comprising any barrier as described herein, whereby currents can cross said barrier due to increasing or decreasing the applied voltage over said barrier.
Yet any combination of any of the embodiments or aspects of the invention can be achieved and such spin-valve structures or methods of production or use will create advantageous devices. Also any of the substrates mentioned above can be an interlayer or a multitude of such interlayers of such substrate materials being deposited on a carrier substrate.